1. Field of the Invention
The invention generally relates to a battery voltage balance apparatus and a battery charge apparatus. More particularly, the invention relates to a battery voltage balance apparatus and a battery charge apparatus which perform the battery voltage detecting and the battery voltage balancing in different time.
2. Description of Related Art
With the development of portable electronic products, the requirement for chargeable battery is gradually increased. The type of chargeable battery is classified into the conventional nickel-cadmium (NiCd) battery, the advanced nickel-metal hydride (NiMH) battery and lithium ion (Li-ion) battery, and the modern Li-Polymer battery. The voltages provided by the different type of chargeable batteries are different, and the operation voltages of the portable electronic products are also different. Accordingly, the manufacturer may couple a plurality of batteries in series as a battery module to provide the desired voltage depending on the operation voltage of the portable electronic product.
When the energy of the batteries in the battery module has been depleted, a battery charger is needed to charge the battery module for next usage. However, the battery capacities of the batteries in the battery module may be different depending on manufacture and usage. For example, a 7.4V Li battery module is formed by two 3.7V Li batteries coupled in series. Before the two batteries are dispatch from the factory, the battery capacities thereof are respectively 80% and 70%. Because over charging will damage the Li battery, the Li battery charger may stop charging the Li battery module when any of the batteries has been fully charged. At this time, the battery capacities of the two batteries may respectively be 100% (the maximum) and 90%. During usage, when any of the battery capacities has fallen down to 0% (the minimum), the battery module can not be used any more. Accordingly, when the battery capacities of the two batteries have respectively fallen down to 10% and 0%, the battery module must be charged before usage.
As known from above, when the battery capacities of the batteries in the battery module are different, the practical electric power of the battery module for usage is determined according to the battery having the lower battery capacity. Besides, when the battery is not used, the battery may self-discharge. In the condition that the self-discharge rate of each battery is different, the battery capacities thereof will be gradually unbalanced, so that the practical electric power of the battery module for usage also gradually decreases as the time goes on, thereby lowering the efficiency of the battery module and shortening the battery life thereof.
Referring to FIG. 1, it shows a digital battery balance controller which is disclosed in the datasheet of ISL9208 by Intersil. A digital battery balance controller 10 includes a battery balance microcontroller 5 and transistor switches S1-S7. The transistor switches S1-S7 are respectively parallel with batteries BAT1-BAT7 through resistors R1-R7. The voltages of the batteries BAT1-BAT7 are converted to digital signals through A/D converters. According to the digital signals corresponding to the batteries BAT1-BAT7, the battery balance microcontroller 5 determines the battery having the highest voltage by an algorithm built inside, and further, turns on the parallel transistor switch corresponding to the battery having the highest voltage. Accordingly, the charge current of each battery can be adjusted based on the voltage of each battery to achieve the function of charge balance.
However, each battery has internal resistance, and when the charge balance is performed, the charge currents of the batteries are different, i.e. the charge current of the battery having the higher battery voltage is smaller. Accordingly, the measurement for battery voltages is not correct. Especially, for the battery module having the larger battery capacity or charging by fast charging technology, the charge currents are relatively large, such that the measurement errors of battery voltages are also larger.